General Chemistry Principles, Patterns, and Applications, 2011

everse process, the reduction of stannous ions (Sn 2+ ) by metallic beryllium, which has a positive value
of E° cell, will occur spontaneously.
b. A MnO 2 is the oxidant (Mn 4+ is reduced to Mn 2+ ), while H 2O 2 is the reductant (O 2− is oxidized to O 2).
We can obtain the standard electrode potentials for the reduction and oxidation half-reactions directly
fromTable 19.2 "Standard Potentials for Selected Reduction Half-Reactions at 25°C".
B The two half-reactions and their corresponding potentials are as follows:
cathode :anode :overall : MnO2( s) + 4H + ( aq) + 2e -
® Mn2 + ( aq) + 2H 2O( l) H 2O2( aq) ® O2( g) + 2H +
( aq) + 2e - MnO2( s) + H 2O2(a
q) + 2H + ( aq) ® O2(g) + Mn2 + ( aq)
+2H 2O( l)E°cathode = 1.22 VE°anode
= 0.70 VE°cell = E°cathode - E°anode = -0.53 V
The standard potential for the reaction is positive, indicating that under standard conditions, it will occur
spontaneously as written. Hydrogen peroxide will reduce MnO 2, and oxygen gas will evolve from the
solution.
Exercise
Use the data in Table 19.2 "Standard Potentials for Selected Reduction Half-Reactions at 25°C" to determine
whether each reaction is likely to occur spontaneously under standard conditions:
a. 2Ce 4+ (aq) + 2Cl − (aq) → 2Ce 3+ (aq) + Cl 2(g)
b. 4MnO 2(s) + 3O 2(g) + 4OH − −
(aq) → 4MnO 4 (aq) + 2H2O
Answer:
a. spontaneous (E° cell = 0.36 V)
b. nonspontaneous (E° cell = −0.20 V)
Although the sign of E°cell tells us whether a particular redox reaction will occur spontaneously under
standard conditions, it does not tell us to what extent the reaction proceeds, and it does not tell us what
will happen under nonstandard conditions. To answer these questions requires a more quantitative
understanding of the relationship between electrochemical cell potential and chemical thermodynamics,
as described inSection 19.4 "Electrochemical Cells and Thermodynamics".
Summary
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